I'm not so sure. They seem to want to downplay the importance of the OGL. Dropping the bomb via Twitter while at Gen Con while they have the chance to speak to a few of the more prominent potential users in person (those who are third party ready and can afford to be at such a show) makes quite a bit of sense.
Have they ever dropped a bomb via Twitter? Ever?
They might drop a link to the bomb site, but that's different. In that case, Twitter is raising awareness. And it's just one of many forms of social media being used to raise awareness of an announcement, and not the announcement itself.
But 160-characters is a little lite for full details of a licence.
I imagine if they were at the position to talk privately with interested companies they wouldn't need to announce anything at all. In the same way they talked about storylines and products. They just say "Hey, we want to talk OGL with you. Sign this NDA."
They don't even need to wait for the Con to do that. A good percentage of the people they work with live in Seatle and could be managed by short drive, or even a Skype call.
If they really wanted to downplay the licence, they wouldn't make it public an all, and just make it invite only.
Really, GenCon is where they used to announce things to the fans, but they stopped doing that, oh, in 2011. Since then there hasn't been a single major convention announcement. Everything has been press releases. There's no reason to think that would change for an OGL, which the fans are unlikely to be interested in. Standing in front of an audience of 700 people and saying something that makes 5 of them excited is remarkably inefficient.